The present invention relates to a concentric optical system which is usable as either an ocular optical system or an imaging optical system. More particularly, the present invention relates to a concentric optical system which has a wide angle of view and high resolution.
In general, a Schmidt system, which is well known as an objective for astronomical telescopes, is arranged as shown in the sectional view of FIG. 23. That is, an aspherical lens LA which is close to a plane-parallel plate is placed at the center of curvature of a concave mirror MC to correct spherical aberration, and a diaphragm D is disposed at the center of curvature to correct coma and astigmatism.
Concentric optical systems, which are represented by the above Schmidt system, are free from coma and astigmatism by virtue of the diaphragm D disposed at the center of curvature of the concave mirror MC. However, since the curvature of field cannot be corrected, a large field curvature occurs. In addition, since the image position lies in front of the concave mirror MC, if a film F or a CCD, for example, is disposed on the image surface, the bundle of incident rays is eclipsed.
It should be noted that U.S. Reissued Pat. No. 27,356 discloses an ocular optical system which, as shown in FIG. 24, uses a semitransparent concave mirror 6 and a semitransparent plane mirror 16 to project an object surface 62 in a distance, and which adopts an arrangement wherein the field curvature produced by the semitransparent concave mirror 6 is corrected by curving the object surface 62. It should be noted that reference numeral 66 in FIG. 24 denotes an exit pupil.
To solve the above-described problems, one type of conventional concentric optical system uses a convex mirror MV, as shown in the sectional view of FIG. 25(a) or 25(b). In this type of concentric optical system, the image surface can be placed behind the concave mirror MC by the convex mirror MV.
However, these conventional concentric optical systems are designed for telescopes or reflecting telephoto lenses, and most of them have a long focal length and a narrow angle of view. There has heretofore been no optical system having a wide angle of view and a small F-number.
Lens systems of wide view angle which are composed of ordinary refracting lenses have the disadvantage that the number of constituent lenses increases and the structure becomes complicated.